1. Field of the Invention
The present invention generally relates to a hydrodynamic gas bearing and manufacturing method thereof.
More particularly, the present invention relates to a hydrodynamic gas bearing supporting a rotator rotating at a high speed and a manufacturing method thereof.
2. Description of the Background Art
Recently, high rotational accuracy as well as high rotational speed have been required of a rotation driving part of a magnetic recording apparatus, for example, a hard disk driver. It has been desired to drive a spindle motor constituting the apparatus mentioned above at high rotation speed and high rotational accuracy. Conventionally, a ball bearing has been used for the bearing part of the motor. The ball bearing, however, generally has a short life in a range of high speed rotation exceeding 10000 rpm, and therefore it has been difficult to attain high rotational accuracy. In order to solve this problem, a so-called hydrodynamic oil bearing using oil as a lubricant has been studied. Even when the hydrodynamic oil bearing is used, however, satisfactory performance is not ensured, as the bearing suffers from the problem of oil leakage or the like.
A gas bearing has been known as a bearing allowing high speed rotation, in which a shaft body and a bearing body are supported in non-contact manner by means of gas such as air. The gas bearing is divided into a hydrostatic gas bearing separately provided with an apparatus for compressing gas, in which the shaft body and the bearing body are supported in non-contact manner by a floating force obtained by the pressure from gas compression, and a hydrodynamic gas bearing in which the shaft body and the bearing body are supported in non-contact manner by a floating force obtained by a pressure derived from an air flow generated by the rotation of the shaft body or the bearing body itself.
The hydrostatic gas bearing supports stable rotation from low speed to high speed. The hydrostatic gas bearing, however, requires a compressing apparatus separate from the body, which means that manufacturing cost is high.
By contrast, in principle, the hydrodynamic gas bearing can be manufactured at a low cost. When the hydrodynamic gas bearing is used, however, a phenomenon called "half speed whirl", which is an unstable vibration, can be sometimes observed at a high speed rotation (see, for example, Gas Bearing by Shinichi Tohgo, Kyoritsu Shuppan (1984)). This phenomenon is such that the shaft is pressed against the bearing surface by centrifugal force to whirl at a frequency of a half number of rotation in the interior of the bearing. When this phenomenon occurs, rotational accuracy degrades at a high speed rotation, and at a low speed rotation, the shaft body and bearing body rotate in contact with each other even in a range of relatively high number of rotation, resulting in wear and shorter life.
In view of these problems, as a method of suppressing the half speed whirl phenomenon at high speed rotation, use of a so-called herringbone bearing, that is, a bearing having V shaped grooves formed on a surface of the shaft body or the bearing body has been proposed. Manufacturing of the herringbone bearing, however, requires complicated and precise processing, which leads to higher manufacturing cost.
Further, as a method of suppressing the half speed whirl phenomenon at a high speed rotation, there is a proposal to make the shape of an outer periphery of a cross section of the shaft body to an approximately triangular shape with rounded corners (see Japanese Patent Laying Open-No. 9-264317, International Publication WO97/41362, and so on). The cross section of the shaft body with outer periphery emphasized is shown in FIG. 6. As can be seen from FIG. 6, the shape of the outer periphery of shaft body 101 is deviated from a complete round 100. In FIG. 6, the deviation between the complete round 100 and the outline representing the shaft body 101 are emphasized for easier understanding. Processing of the outer periphery of the shaft body under control to provide not circular but approximately triangular cross sectional shape requires higher manufacturing cost.
Further, in order to suppress the half speed whirl phenomenon at a high speed rotation, formation of a groove extending parallel to the axial line on the outer periphery of the shaft body (hereinafter referred to as "longitudinal groove") has been proposed (see Japanese Patent Laying-Open No. 58-163818, for example). It is possible to form the longitudinal groove at a low manufacturing cost and it is very effective to suppress the half speed whirl phenomenon at a high speed rotation. For example, longitudinal grooves are formed on the outer peripheral surface of the shaft body as shown in FIG. 7. As can be seen from FIG. 7, longitudinal grooves 102 are formed extending in the direction of the axial line on the outer peripheral surface of shaft body 101.
There arises a problem, however, that if the cross sectional shape of the shaft body is not a complete round, "floating rotational number" increases extremely dependent on the relation between the cross sectional shape and the positions of the grooves formed on the outer peripheral surface of the shaft body. Here, "floating rotation number" refers to the number of rotation at the time when the shaft body and the bearing body makes a transition from a contact state to a non-contact state when the rotation of the shaft body or the bearing body is started, or the number of rotation at a time when the shaft body and the bearing body makes a transition from the non-contact state to a contact state when the speed of rotation is reduced from the high speed, normal rotating state to stop rotation. Thus, there has been such a problem that it is impossible to switch between the contact state and the non-contact state of the shaft body and the bearing body at a low number of rotation, which means that the shaft body and the bearing body are kept in contact with relatively high number of rotation at the start or stop of rotation, resulting in wear and shorter life.
FIG. 8 represents a cross section of the hydrodynamic gas bearing suffering from the above described phenomenon, in emphasized manner. As can be seen from FIG. 8, shaft body 101 has a non-circular, approximately triangular cross sectional shape. The outer periphery of shaft body 101 has three protruding portions a1, a2 and a3. Further, longitudinal grooves 102a, 102b and 102c are formed on the outer periphery of shaft body 101. A cylindrical bearing body 200 is placed opposing, with a space in radial direction, to the outer periphery of shaft body 101 having such a cross sectional shape.
The outline representing the cross sectional shape of shaft body 101 circumscribes a complete round 100a and inscribes a complete round 100b. Here it is assumed that deviation .delta. between the circumcircle 100a and incircle 100b of the outline representing the cross sectional shape of shaft body 100, that is, out-of-roundness is about 0.2 .mu.m. Even when the cross sectional shape of shaft body 101 has this very small out-of-roundness, the floating rotational number increases extremely high when bearing body 200 is rotated in a direction presented by the arrow R, causing wear and resulting in shorter life.
The reason for this is that longitudinal grooves 102c, 102b and 102a are positioned behind respective protruded portions a1, a2, and a3 of shaft body 101 with respect to the direction of rotation R of bearing body 200. In the conventional structure of the hydrodynamic gas bearing, it has been difficult to form longitudinal grooves on the outer periphery of the shaft body with the position of the longitudinal grooves not in the above described relation with the position of the protruding portions in its cross section if the cross sectional shape of the shaft body is not a complete round. In order to form the longitudinal grooves on the outer periphery of the shaft body to satisfy the prescribed relation, very high processing accuracy must be ensured, which leads to higher manufacturing cost.